Custom stored procedures with variable number of parameters

[badrishc]

Support custom commands (stored procedures) with a variable number of parameters. This can be used to implement commands such as a custom MSET variation that takes expiration as an argument.

Example of MSETPX is included in the PR here: https://github.com/microsoft/garnet/blob/ecbddd2c09b196cb19cdb67005f7ccc92192c214/main/GarnetServer/Extensions/MSetPx.cs

Example usage: set 5 key-values pairs with an expiration of 60 seconds (60000 milliseconds):

MSETPX 60000 key1 value1 key2 value2 key3 value3 key4 value4 key5 value5

Contributes to #366

[badrishc]

Added example for multi-get, where values match a specified prefix. Example:

127.0.0.1:3278> mset key1 com/value1 key2 org/value2 key3 edu/value3 key4 com/value4
OK
127.0.0.1:3278> mgetifpm com key1 key2 key3 key4
1) "key1"
2) "com/value1"
3) "key4"
4) "com/value4"

Stored proc is here: main/GarnetServer/Extensions/MGetIfPM.cs