feat(Algebra/Spectrum): add the second resolvent identity

[godofecht]

Adds spectrum.resolvent_sub_resolvent:

For a b : A in an R-algebra and r in the resolvent set of both,
resolvent a r - resolvent b r = resolvent a r * (a - b) * resolvent b r.

Companion to spectrum.resolvent_eq.

[github-actions]

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[github-actions]

PR summary d163059006

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ resolvent_sub_resolvent

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.