Control included quota and storage providers via build tags

[robstradling]

#3644 (comment) noted that the log-signer and log-server binaries are huge, due in part to the presence of multiple quota and storage providers. This PR enables providers to be included selectively via Go build tags.

By default and for backwards compatibility, all providers are included:

go build && ls -sh trillian_log_server
62M trillian_log_server*

Specify one build tag to include just one provider:

go build -tags=cloudspanner && ls -sh trillian_log_server
54M trillian_log_server*
go build -tags=crdb && ls -sh trillian_log_server
37M trillian_log_server*
go build -tags=mysql && ls -sh trillian_log_server
34M trillian_log_server*
go build -tags=postgresql && ls -sh trillian_log_server
40M trillian_log_server*

Specify multiple, comma-separated build tags to include multiple providers. e.g.,:

go build -tags=mysql,cloudspanner && ls -sh trillian_log_server
55M trillian_log_server*
go build -tags=cloudspanner,crdb,mysql,postgresql && ls -sh trillian_log_server
62M trillian_log_server*

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

[phbnf]

/gcbrun

[phbnf]

Looks neat, thanks. Do you mind adding an entry in CHANGELOG.md, and something on the Working on the Code section to both publicize this feature and explain what needs to be done when someone adds a new backend storage?

[robstradling]

Hi @phbnf. Done.

[phbnf]

/gcbrun

[phbnf]

Thanks @robstradling !