feat(GroupTheory/ResiduallyFinite): define residually finite groups

[tb65536]

This PR defines residually finite groups, provides some basic API, and gives the connection with profinite completion.

Some to_additivizing is still waiting on #34784.

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[github-actions]

PR summary cde919c85c

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Topology.Algebra.Category.ProfiniteGrp.Completion	1666	1669	+3 (+0.18%)

Import changes for all files

Files	Import difference
Mathlib.Topology.Algebra.Category.ProfiniteGrp.Completion	3
Mathlib.GroupTheory.ResiduallyFinite (new file)	851

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Declarations diff

+ AddGroup.ResiduallyFinite
+ ResiduallyFinite
+ etaFn_injective_iff_residuallyFinite
+ eta_injective_iff_residuallyFinite
+ instance [Finite G] : ResiduallyFinite G
+ mem_toSubgroup_iff
+ residuallyFinite_def
+ residuallyFinite_iff_exists_finiteIndex
+ residuallyFinite_iff_exists_finiteIndexNormalSubgroup
+ residuallyFinite_iff_forall_finiteIndex
+ residuallyFinite_iff_forall_finiteIndexNormalSubgroup

You can run this locally as follows

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for scripts/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).