Minor webhook and http component improvements

[andrewsayre]

Description:

1. In the webhook component, adds the ability to generate a webhook path portion only (`async_generate_path'). This will be used by the SmartThings component (Add SmartThings component and switch platform #20148) so it does not need to hard-code the webhook path format (or alternatively, strip the base_url, url parse..., etc...).

2. Also updates the http component to ensure that the base_url does not end up in a trailing slash. This could happen if the user accidentally enters a trialing slash on base_url in config which then results in generating webhook paths with an unnecessary slash after the host/port (i.e. http://localhost:8123//api/webhook/...)

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.