Fix mamba environment option

[dstansby]

I'm attempting to fix #1353. Currently this involves:

• adding some extra docs
• adding an import guard to check that libmambapy is installed
• making the mamba plugin discoverable by asv (by importing it in plugins.__init__.py)

However, I'm not convinced the mamba option has ever worked... The mamba Python API does not contain mamba.api.MambaSolver(), which was added as an import in #1238. @HaoZeke do you know any more about this import and what package needs to be installed to access it?

[HaoZeke]

@HaoZeke do you know any more about this import and what package needs to be installed to access it?

I will take a closer look after next week (am traveling). It was certainly available at the time of that PR, though libmambapy has changed quite a bit over the past year or so.

[HaoZeke]

So there used to be a mamba.api which shipped a default implementation of MambaSolver as noted in an older fork. It was removed in December of 2023. However it has been removed so I'll setup a different default implementation for us (boa moves far too quickly and is too heavy to be a dependency).

[HaoZeke]

This shouldn't be required? (plugins are auto-discovered, failures mean they are dropped from valid options)

[HaoZeke]

Rather than explicitly raise from here the current approach is to not let mamba be a valid option for an environment if the import fails.

The reason behind this is to allow for external plugins (it is also why we don't want to have an explicit list of allowed values).

[HaoZeke]

Suggested change

	but needs a newer Python version (3.8 or greater) and the ``libmambapy``
	package must be installed to provide access to the mamba Python API.
	but needs a newer Python version (3.8 or greater) and the ``libmambapy``
	package must be installed (typically through ``conda``) to provide access to the mamba Python API.

Clarification since libmambapy is not present on PyPI.

[HaoZeke]

Closing in favor of #1372 and #1373.

[dstansby]

Fab, thanks for picking this up!