x/tools/internal/mcp: add custom HTTP client overload

[andrey-star]

Previously, the MCP client relied on "http.DefaultClient"
which didn't allow for customization.
Here I'm adding an overload that accepts an "http.Client" as parameter.

Fixes golang/go#74108

[gopherbot]

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Message from Andrey Starodubtsev:

Patch Set 2:

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Patch Set 3:

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Message from Andrey Starodubtsev:

Patch Set 3:

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Message from Robert Findley:

Patch Set 3:

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[andrey-star]

Closed by 84508c7