feat(client): Add cobra's context to clientCtx

[amaury1093]

Description

With Textual #13646, we pass a context.Context argument to a lot of signing functions. In a previous PR, #13747, we set this argument to context.TODO(), waiting for other pieces of Textual to be finished.

Now that Textual's implementation is finished, we should replace these instances of context.TODO(). There are 2 proposals:

1. replace with context.Background(): easy, harmless, works with Textual... but feels incorrect, since we're creating a new context.Context on each sign.
2. replace with cobra command's context.Context: more correct, needs some plumbing to pass it around to all functions.

This PR implements 2.

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[julienrbrt]

lgtm!

[tac0turtle]

nit: rename to Context, or is there a reason it called cmdContext?

[amaury1093]

The struct is already called Context. I thought being explicit here was useful to know what we're talking about. Can rename to simply Context if other people also feel the same.