Add new attribute: inhibit_automatic_where_clause

[vi]

Allows to opt out of where clause, in cases where it gets in the way.

My use case:

#[delegatable_trait]
trait SomeTrait{}

#[derive(ambassador::Delegate)]
#[delegate(SomeTrait,inhibit_automatic_where_clause="true")]
pub struct DynSomeTrait(pub Box<dyn SomeTrait + Send>);

It allows user to request structural typing (like with target="self"), but with other targets instead of self.

If this feature is welcome in ambassador, I can also provide in this pull request:

• Similar attribute for impl delegation
• Documentation
• Test

[vi]

@dewert99, Done.

Some changes:

• I changed inhibit_automatic_where_clause="true" to automatic_where_clause="false" to avoid double negation in API. Internal name stays inhibit_automatic_where_clause due to Default implementation.
• I have added a test ensures that a workaround of attaining dyn delegation this without automatic_where_clause="false" works and will not be accidentally broken (i.e. by implementing automatic where clauses for #[delegate_to_methods]) without a major semver increase.

[dewert99]

This looks great do you think you could run rustfmt and then I can pull this