Add FungibleAdapter (#2684)

In the move from the old `Currency` traits to the new `fungible/s`
family of traits, we already had the `FungiblesAdapter` and
`NonFungiblesAdapter` for multiple fungible and non fungible assets
respectively. However, for handling only one fungible asset, we were
missing a `FungibleAdapter`, and so used the old `CurrencyAdapter`
instead. This PR aims to fill in that gap, and provide the new adapter
for more updated examples.

I marked the old `CurrencyAdapter` as deprecated as part of this PR, and
I'll change it to the new `FungibleAdapter` in a following PR.
The two stages are separated so as to not bloat this PR with some name
fixes in tests.

---------

Co-authored-by: command-bot <>

prdoc/pr_2684.prdoc

@@ -0,0 +1,14 @@
+title: Add XCM FungibleAdapter
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      A new AssetTransactor has been added to xcm-builder: FungibleAdapter.
+      It's meant to be used instead of the old CurrencyAdapter for configuring the XCM executor
+      to handle only one asset.
+
+crates:
+  - name: "xcm-builder"
+
+migrations: []
+host_functions: []