[API Proposal]: Add an equivalency-options overload to WithBodyEquivalentTo

## Motivation

`WithBodyEquivalentTo(T expected, ...)` runs a full equivalency comparison with no way to configure it. That breaks down as soon as the request body contains anything non-deterministic:

- a client-generated `Guid` or correlation id
- a `DateTime` stamped at send time
- a field you simply do not care about

FluentAssertions users expect `BeEquivalentTo` to take an options lambda, and its absence here forces people to abandon the assertion and fall back to `WithBodyHavingProperty` one field at a time — which loses the nice diff output.

## Proposed API

```csharp
requests.Should().ContainRequestFor("/api/users")
.WithBodyEquivalentTo(expected, options => options
.Excluding(x => x.Id)
.Using(ctx => ctx.Subject.Should().BeCloseTo(ctx.Expectation, 1.Seconds()))
.WhenTypeIs());
```

Signature mirroring FluentAssertions:

```csharp
AndWhichConstraint WithBodyEquivalentTo(
T expected,
Func, EquivalencyOptions> config,
string because = "",
params object[] becauseArgs);
```

## Notes

- The options type differs between FluentAssertions 7 and 8 (`EquivalencyAssertionOptions` vs `EquivalencyOptions`), so this needs the existing `#if FA8` treatment in `Shared/HttpMockAssertionExtensions.cs`.
- The existing parameterless overload stays, delegating with a no-op config.
- Same consideration would apply to a response-side `WithResponseBodyEquivalentTo` if #75 is implemented.

Start with Shared/HttpMockAssertionExtensions.cs and the existing WithBodyEquivalentTo entry point. Check the current #if FA8 treatment and the FluentAssertions option types for versions 7 and 8. Done means the new options overload supports the proposed configuration, the parameterless overload still works through a no-op configuration, and both compatibility paths build.

Written by the indexing model from the issue text.