Add ToLower & ToUpper template functions for config.TemplatedStringAsIdentifier

[ulucinar]

Description of your changes

This PR adds ToLower and ToUpper template functions that can be used in the template body passed to config.TemplatedStringAsIdentifier. With these template functions, it will be possible to specify an external-name configuration such as the following:

config.TemplatedStringAsIdentifier("", "arn:aws:network-firewall:{{ .setup.configuration.region }}:{{ .setup.client_metadata.account_id }}:{{ .parameters.type | ToLower }}-rulegroup/{{ .external_name }}"),

, where the value of the type parameter will be converted to lower case and used as a component while constructing the Terraform ID.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Tested manually in the context of crossplane-contrib/provider-upjet-aws#661.

[sergenyalcin]

Thanks @ulucinar LGTM!