ARI: Added paths to get and set multiple channel variables.

[bkford]

Two new paths exist for ARI to get and set multiple channel variables at
the same time. This is done via GET and POST like the single get and set
variable equivalents. Leading and trailing whitespace will be stripped
from the variable names for both paths. When setting variables, the
values will be read as-is, whitespace included. GET takes in a single
string with comma-separated values, while POST takes in a dictionary of
key value pairs. The code follows the same paths as when setting
multiple variables when originating a channel via ARI.

UserNote: Added new ARI paths for getting and setting multiple channel
variables at a time. For GET, this takes in a single string of
comma-separated variable names, while POST takes in a dictionary of key
value pairs. The behavior is the same as passing in variables when
originating a channel.

[bkford]

cherry-pick-to: 23
cherry-pick-to: 22
cherry-pick-to: 20

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[gtjoseph]

In the commit message, I don't understand what the following means...

You can also use getChannelVars and setChannelVars in your applications.

[bkford]

NOTE
One concern I have is the locking, so extra eyes on that would be greatly appreciated. The main concerns are:

1. When setting multiple variables, ast_set_variables is called and locks the channel. It eventually gets to pbx_builtin_setvar_helper, which ALSO locks the channel. Originating a channel also does this.
2. When setting a single variable (the original, existing path), it calls a different function: stasis_app_control_set_channel_var. This does NOT lock the channel before calling, but it ALSO eventually gets to pbx_builtin_setvar_helper.
3. In the original, existing get variable function, it makes a note of not locking when reading from the channel due to potential locking issues, especially with autoservice dialplan functions. It may not have anything to do with a write, but it's a concern I had.

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Resolved

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master-app_dial: FAILED TEST: apps/queues/transfer/caller_attended
master-ari2: FAILED TEST: rest_api/channels/channel_variables/get_set_multiple

Need failing testsuite test corrected.

[gtjoseph]

Need failing get_set testsuite test to pass.

Resolved

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master-pjs2: FAILED TEST: channels/pjsip/transfers/attended_transfer/nominal/callee_local

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master-pjs5: FAILED TEST: channels/pjsip/subscriptions/rls/lists_of_lists/nominal/mwi/batched

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[gtjoseph]

Moving to DRAFT because @mbradeen has a related PR coming and we want to make sure they're in sync before merging.

[bkford]

Included in #1917