fix: submit JCL from stdin

[traeok]

What It Does

• Implement Timothy's change in CLI plug-in for "submit JCL from stdin handler"
• golang: Fix HandleSubmitJclRequest function to allow passing JCL encoding
• c: Fix handle_job_submit_jcl function so it
  • converts input data from UTF-8 to given encoding before submitting JCL
  • accepts raw bytes from stdin when stdout isn't a TTY (scenario: user invokes zowex job submit-jcl in a shell)
• c: Rename ZJB_UNKNWON_RC -> ZJB_UNKNOWN_RC and re-define constant as empty string (fixes bug in VSCE)

How to Test

Client:

• Open a data set member in Zowe Explorer containing JCL
• Right-click in the editor -> "Submit as JCL"
• See the notification in the bottom-right? Click the "JOBXXXXX" hyperlink in the notification to reveal it in the Jobs tree.

zowex:

• cat JCL_CONTENTS.txt | ./zowex job submit-jcl where JCL_CONTENTS.txt is:

//DONOTHING JOB (IZUACCT),'DONOTHING',CLASS=A,MSGCLASS=A,MSGLEVEL=(1,1),NOTIFY=&SYSUID

//STEP01 EXEC PGM=IEFBR14

//SYSPRINT DD SYSOUT=*
//SYSOUT DD SYSOUT=*
//SYSIN DD DUMMY

Review Checklist
I certify that I have:

• tested my changes
• added/updated automated tests
• updated the changelog
• followed the contribution guidelines

[t1m0thyj]

LGTM, thanks @traeok!