gopls/internal/golang: add extract interface code action

[martskins]

This PR adds a code action to extract an interface from a concrete type on a struct field or function parameter.

Fixes golang/go#65721

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 3786889) has been imported to Gerrit for code review.

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[gopherbot]

Message from Robert Findley:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Robert Findley:

Patch Set 2:

(1 comment)

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[gopherbot]

This PR (HEAD: 9c9c1b0) has been imported to Gerrit for code review.

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