Add route attributes - MTU, AdvMSS, Priority

[alopintsev]

Extend route attributes to make route selection available based on route priority and enable per-route MTU settings.
It's will be helpfull in multi-CNI environment, and networks, that support jumbo-frame.

Fixes: #1004

[squeed]

@alopintsev looks reasonable. However, we will need to update the specification as well, as these are defined types. Thanks!

[coveralls]

coverage: 71.566% (-0.007%) from 71.573%
when pulling 4a0bccb on alopintsev:add-route-attributes
into 93878e6 on containernetworking:main.

[alopintsev]

@alopintsev looks reasonable. However, we will need to update the specification as well, as these are defined types. Thanks!

I've made changes in specification, would you mind to check it?

[squeed]

Looks good, thanks!

[squeed]

FYI @aojea / @Joffref, who had also looked at this.

[aojea]

LGTM