SPIFFE_ENDPOINT_SOCKET env support for spire-agent

[kfox1111]

Enables specifying SPIFFE_ENDPOINT_SOCKET to the non run subcommands to the spire-agent

partially-fixes: https://github.com/spiffe/spire/issues/5770

[kfox1111]

SPIFFE_ENDPOINT_SOCKET needs to support unix:// on the front according to: https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE_Workload_Endpoint.md#4-locating-the-endpoint

But this patch doesn't work with that currently.

[kfox1111]

Have run into a few problems with this, and a bigger config question/issue. Still working through it.

[sorindumitru]

@kfox1111, do you still want us to keep this issue around? I think the solution for this will likely be replaced by what was discussed in https://github.com/spiffe/spire/issues/6247.

[kfox1111]

I'm good closing this in favor of #6247