Optionalizes `remotezip` Package

[SpicyGarlicAlbacoreRoll]

• makes remotezip package an optional dependency under "extras"
• warning raised when not installed while attempting to use remotezip() methods

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Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a5f017e) 89.79% compared to head (003663e) 89.61%.

Files	Patch %	Lines
asf_search/download/download.py	50.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #255 +/- ## ========================================== - Coverage 89.79% 89.61% -0.19% ========================================== Files 54 54 Lines 2028 2031 +3 ========================================== - Hits 1821 1820 -1 - Misses 207 211 +4 ``` | [Flag](https://app.codecov.io/gh/asfadmin/Discovery-asf_search/pull/255/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=asfadmin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/asfadmin/Discovery-asf_search/pull/255/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=asfadmin) | `89.61% <55.55%> (-0.19%)` | :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=asfadmin#carryforward-flags-in-the-pull-request-comment) to find out more.

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