Exposed the dirichlet_multinomial distribution

[chvandorp]

In a recent PR, I added the Dirichlet-multinomial distribution to the Stan Math library. This PR exposes the distribution to the user.

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: https://github.com/stan-dev/docs/pull/693
  • [ ] OR, no user-facing changes were made

Release notes

Added the dirichlet_multinomial distribution.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Merging #1389 (073e29f) into master (6de5732) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1389 +/- ## ======================================= Coverage 89.93% 89.93% ======================================= Files 63 63 Lines 10706 10707 +1 ======================================= + Hits 9628 9629 +1 Misses 1078 1078 ``` | [Files](https://app.codecov.io/gh/stan-dev/stanc3/pull/1389?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev) | Coverage Δ | | |---|---|---| | [src/middle/Stan\_math\_signatures.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1389?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL21pZGRsZS9TdGFuX21hdGhfc2lnbmF0dXJlcy5tbA==) | `98.15% <100.00%> (+<0.01%)` | :arrow_up: |