Add s3 server-side encryption and decryption with customer provided key

[liortamari]

I would like to add support for AWS S3 backup/restore using server-side encryption and decryption with customer provided key.

[deepthi]

Welcome @liortamari! Can you fix the DCO? Instructions here: https://github.com/vitessio/vitess/pull/7088/checks?check_run_id=1475698865

[liortamari]

@deepthi Thank you.
I updated the DCO.
I am also working on adding some unit tests.

[deepthi]

@deepthi Thank you.
I updated the DCO.
I am also working on adding some unit tests.

That's great! We'll ask some people to review this who know better than I do.

[aquarapid]

Could we change this to avoid adding a new tablet option, and be more consistent, by just making sse_c a new option value for -s3_backup_server_side_encryption? Could be something like:

-s3_backup_server_side_encryption sse_c:/path/to/key/file

Rest seems fine.

[liortamari]

@aquarapid Thank you.
I applied this change, indeed it looks more consistent now.

[ajm188]

This is purely opt-in, so existing users shouldn't notice, and the new feature looks correct to me. +1

Requested changes have been made.