Support for Yandex Cloud KMS

[luc-lynx]

This PR introduces support for Yandex Cloud KMS.

Some notes:

• Yandex Cloud SDK doesn't have credentials discovery like AWS SDK, so I implemented an approach when a user either passes a file name with credentials or relies on wal-g attempt to get an access token from virtual machine metadata service. AWS SDK uses similar approach.
• AES-256 is used to encrypt Data Encryption Key.
• In contrast to AWS KMS, Data Encryption Key is generated by Yandex Cloud KMS.
• Tests use a mocked version of key and don't make api calls to Yandex Cloud KMS. AWS KMS support uses the same approach. However in my opinion it's better to come up with tests for AWS KMS and YC KMS.

[x4m]

Looks great, thanks!

[x4m]

Is the operation of obtaining key time-consuming?
I mean we are doing wal-push in PG for each 16Mb file. Will not reading key be a bottleneck?

[luc-lynx]

Is the operation of obtaining key time-consuming?

Just an API call to YC KMS. In case of AWS KMS it also requires one API call. On the KMS side it's just one encryption operation, so it shouldn't be slow.

I mean we are doing wal-push in PG for each 16Mb file.

Does it mean that wal-g binary is executed on every 16Mb chunk?

[x4m]

Does it mean that wal-g binary is executed on every 16Mb chunk?
Yes

[x4m]

Thanks, pushed!