Add the ability to do `--credential-override` multiple times

[g-linville]

Rather than making the user specify all credential overrides in a single --credential-override argument, we want to support the ability to specify it multiple times, like this:

gptscript --credential-override toolA:MY_VAR=value --credential-override toolB:MY_OTHER_VAR=value script.gpt

[g-linville]

@sangee2004 this is ready for testing.

[sangee2004]

Tested with gptscript - version v0.0.0-dev-3b38281a-dirty

Able to do -credential-override multiple times for overriding multiple credentials.

1. Execute the following script and set the values for creds to mytest1(credA) and mytest2(credB)

cred: github.com/gptscript-ai/credential as credA with VAR_A as env and "Please enter your cred for A" as message
cred: github.com/gptscript-ai/credential as credB with VAR_B as env and "Please enter your cred for B" as message

#!/usr/bin/env bash

echo "A: $VAR_A"
echo "B: $VAR_B"

2. Execute the script to Override credential values using - gptscript --disable-cache --credential-override credA:VAR_A=mytest1override --credential-override credB:VAR_B=mytest2override credential.gpt

Environment variables get overridden as expected.

gptscript --disable-cache --credential-override credA:VAR_A=mytest1override  --credential-override credB:VAR_B=mytest2override credential.gpt
17:33:22 started  [main]
17:33:22 sent     [main]
         content  [1] content | A: mytest1override
         content  [1] content | B: mytest2override
         content  [1] content | 
17:33:22 ended    [main] [output=A: mytest1override\nB: mytest2override]

OUTPUT:

A: mytest1override
B: mytest2override

3. Execute the script with no override

Values set in the credential entries get used as expected.

gptscript --disable-cache credential.gpt 
17:34:56 started  [main]
17:34:56 sent     [main]
         content  [1] content | A: mytest1
         content  [1] content | B: mytest2
         content  [1] content | 
17:34:56 ended    [main] [output=A: mytest1\nB: mytest2]

OUTPUT:

A: mytest1
B: mytest2