Storage: add service api to read storage object in to output stream using direct download

[ajaaym]

towards #3929

Note: This pr just adds the service api. There would be follow up pr to this one which would add public api to download blob as a file or in to output stream using this service api.

[codecov]

Codecov Report

Merging #5525 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #5525 +/- ##
============================================
- Coverage 50.86% 50.85% -0.01% + Complexity 24193 24192 -1 
============================================
Files 2270 2270 Lines 230115 230151 +36 Branches 25009 25015 +6 ============================================
- Hits 117043 117041 -2 - Misses 104435 104472 +37 - Partials 8637 8638 +1

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/cloud/storage/spi/v1/StorageRpc.java	62.85% <ø> (ø)	0 <0> (ø)	⬇️
...om/google/cloud/storage/spi/v1/HttpStorageRpc.java	1.81% <0%> (-0.04%)	1 <0> (ø)
...ud/storage/contrib/nio/testing/FakeStorageRpc.java	59.21% <0%> (-5.39%)	42 <0> (ø)
...able/gaxx/reframing/ReframingResponseObserver.java	88.99% <0%> (-1.84%)	29% <0%> (-1%)

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[ajaaym]

@frankyn@JesseLovelace can you please review?

[astroyang]

Is there an estimated time for the next step of adding a public api to download blob using this? @ajaaym

[ajaaym]

@astroyang#5791 is merged yesterday.