[ssl] Add support for client side certificate reloading

[euroelessar]

grpc-core part of #15735

[euroelessar]

@jiangtaoli2016 @mehrdada Please have a look, I'm not that certain about the provided API and comments are definitely welcome.

[jiangtaoli2016]

@euroelessar You may be interested in this gRFC grpc/proposal#98. @yihuazhang from my team is working new grpc TLS credential API that supports both SPIFFE and HTTPS semantics, supports credential reloading for both client/server, support server authorization, and so on.

We are hoping to migrate to this new API rather than adding burden to old SSL credential API which is hard to maintain.

[euroelessar]

@jiangtaoli2016 The proposal looks promising, but what is an expected timeline for it to be approved and implemented?

[jiangtaoli2016]

@euroelessar We expect to finish the implementation (grpc core code) by end of Oct.

[euroelessar]

The proposed timeline looks reasonable, thanks. I'll postpone/abandon the PR in the meantime.

[jiangtaoli2016]

Thank you @euroelessar for your patience.

[BACtaki]

@jiangtaoli2016 Is there an update on the new API changes?

[yihuazhang]

@BACtaki #16614 is currently under review, that achieves the purpose.