feat: Add custom callback variant of LspRequestCustom

[blami]

This change allows to provide callback function to custom requests.

Builds on #640 to accommodate scenarios where response is needed from server but cannot be handled by custom handler (as there's no method field).

[blami]

I was wondering if it would be any useful to have also sync and async variant of RequestCustom (e.g. RequestCustomAsync). With these two virtually any missing/niche functionality can be done as custom request.

I was also thinking of providing noop g:LspRequestCustomReply (basically same as WorkspaceExecuteReply except scope) that can be used if caller does not care about response.

Let me know what do you think and I can make those changes.

[blami]

@jclsn Thanks for review, I agree with you. I like approach with *Cb() callback variant. Also I changed the non-callback to just call callback variant providing original WorkspaceExecuteReply so that if logic ever changes fix will be in single place.

The only other option I can think of is to have variadic argument and check for emptyness but that feels too fragile and obscure to me.

Let me know what do you think now.

[jclsn]

Looks good to me, but in the end @yegappan needs to approve.