RFC: How Cargo "features" should be used in the SDK and runtime crates

[Velfi]

This is an early version of the features RFC in case I get hit by a bus. Feel free to comment.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[github-actions]

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A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Server Test

[github-actions]

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Server Test (ignoring whitespace)

[github-actions]

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Server Test

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Server Test (ignoring whitespace)

[github-actions]

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-0.88%	79942.04	80651.2
Total requests	-0.91%	7199438	7265467
Total errors	NaN%	0	0
Total successes	-0.91%	7199438	7265467
Average latency ms	35.53%	1.03	0.76
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-12.13%	23.69	26.96
Stdev latency ms	34.35%	1.76	1.31
Transfer Mb	-0.91%	748.39	755.25
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-0.96%	85310.72	86137.1
Total requests	-0.96%	7684683	7759388
Total errors	NaN%	0	0
Total successes	-0.96%	7684683	7759388
Average latency ms	-2.31%	1.27	1.3
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-14.53%	23.94	28.01
Stdev latency ms	-1.32%	2.24	2.27
Transfer Mb	-0.96%	798.83	806.59
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-15.90%	60455.62	71888.28
Total requests	-15.91%	5446135	6476288
Total errors	NaN%	0	0
Total successes	-15.91%	5446135	6476288
Average latency ms	13.79%	0.66	0.58
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	3.32%	17.74	17.17
Stdev latency ms	12.99%	0.87	0.77
Transfer Mb	-15.91%	566.13	673.21
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Server Test

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	2.90%	45047.74	43776.55
Total requests	2.90%	4055164	3940791
Total errors	NaN%	0	0
Total successes	2.90%	4055164	3940791
Average latency ms	-5.19%	0.73	0.77
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-2.66%	18.32	18.82
Stdev latency ms	-18.97%	0.47	0.58
Transfer Mb	2.90%	421.54	409.65
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0