Send project-specific `User-Agent` HTTP header

[jakob-keller]

Description of Change

Includes aiobotocore identifier and version in default User-Agent HTTP header.

Assumptions

Customizing the default User-Agent HTTP header is beneficial for this project and AWS, while not impacting users. See #853 for discussion.

Checklist for All Submissions

• [x] I have added change info to CHANGES.rst
• [ ] If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • [ ] Detailed description of issue
  • [ ] Alternative methods considered (if any)
  • [ ] How issue is being resolved
  • [ ] How issue can be reproduced
• [x] If this is providing a new feature (can be provided via link to issue with these details): closes #853
  • [x] Detailed description of new feature: Add method AioSession._set_user_agent_for_session() which is called in AioSession.__init__() and mimics the behaviour taken by aws-cli.
  • [ ] Why needed
  • [x] Alternatives methods considered (if any): see #853 and #870

Checklist when updating botocore and/or aiohttp versions

• [ ] I have read and followed CONTRIBUTING.rst
• [ ] I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• [ ] I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (743c26f) 86.25% compared to head (a63e037) 86.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1075 +/- ## ========================================== - Coverage 86.25% 86.23% -0.03% ========================================== Files 60 60 Lines 5858 5870 +12 ========================================== + Hits 5053 5062 +9 - Misses 805 808 +3 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1075/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1075/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `86.23% <100.00%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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