[C++][Plasma] Add const constraint when parsing data

[asfimport]

Input data for plasma protocol.h/protocol.cc should be const.

Reporter: Siyuan Zhuang / @suquark
Assignee: Siyuan Zhuang / @suquark

PRs and other links:

• GitHub Pull Request #6579

_{Note: This issue was originally created as ARROW-8072. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 6579
#6579