Add gptel-register-tool function

[ultronozm]

• gptel.el (gptel-register-tool): New function to register a tool with
  gptel, replacing any existing tool with the same name.

Following
skissue/llm-tool-collection#2 (comment)

[karthink]

@ultronozm I'm a little confused about this. gptel-make-tool already registers the tool in gptel--known-tools, so the only thing this function does is to add a tool to gptel-tools?

[ultronozm]

@karthink I had thought that `gptel-make-tool` was a dumb struct constructor that did not register the tool, but I see now that it does. The intent of this PR was to provide a hook via which evaluating an `llm-tool-collection-deftool` macro would update the tool in `gptel`, to make it easier to develop and test tools. I can't check right now whether `gptel-make-tool` already achieves this due to the issue noted at skissue/llm-tool-collection#3, but if not, then I guess `gptel-make-tool` would be the right place to address that. Thus, I think you can safely close this PR. Thanks!