Add `httpx.Mounts` transport class.

[lovelydinosaur]

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Discussed in https://github.com/encode/httpx/issues/947#issuecomment-1881173378

This PR adds the Mounts and AsyncMounts transport classes.

Currently in draft... switch mounts=... out of the Client class directly.

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Here's the synopsis...

We currently have mounts=... behaviour backed directly into the client, like so...

client = httpx.Client(mounts=...)

However it'd be neater to keep that complexity encapsulated within a transport class that supports routing, so...

transport = httpx.Mounts(...)
client = httpx.Client(transport=transport)

At this point in time the pull request replicates the mounts=... functionality in a transport class, but doesn't yet remove it from the client. A follow-up would deal with removing it.