[REQ] Allow passing a config directly to the signer

[cpuguy83]

Details

Signers will often need some configuration passed along from the client (or alternatively they'd need to be built into the signer image, which isn't ideal for images).

Today a client can use --build-context to pass the signer a config, but the signer will need to know a specific key to look for. Also, these deals strictly with llb.State's, so the signer will also need to know which file(s) in the passed in data is what. Again, this can be convention, but can also be difficult to work with due to having to craft a context to look exactly like the signer is expecting.

I think we should allow the spec to declare what the config should be. We should be able to take advantage of the Source type to declare this in the spec.

Basically, instead of requiring the signer to understand all the docker-isms, Dalec can gather that and pass it in directly to the frontend as LLB.

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Dalec itself could do some extra work here even without custom config to see if there is a build context with some predefined key (e.g. singer-config and <distro>-signer-config comes to mind) and automatically pass that along as a frontend input to the signer so that the signer doesn't need to work this stuff out.