Handle both legacy and current @type values in forward messages

Currently, agency expects and uses legacy `@type` attribute for forward messages:
```
did:sov:BzCbsNYhMrjHiqZDTUASHg;spec/routing/1.0/forward
```

However, as per aries spec (see [here](https://github.com/hyperledger/aries-rfcs/blob/08653f21a489bf4717b54e4d7fd2d0bdfe6b4d1a/concepts/0020-message-types/README.md#aries-core-message-namespace)), a different value should be used for the attribute:
```
https://didcomm.org/routing/1.0/forward
```

Agency should support both formats for some time in order to remain backwards compatible.

Analogous change was already implemented in aries-vcx (see [here](https://github.com/hyperledger/aries-vcx/pull/341)).