MINOR: ssl/ech: key management via stats socket

MINOR: ssl/ech: key management via stats socket

This patch extends the ECH support by adding runtime CLI commands to
view and modify ECH configurations.

New commands are added to the HAProxy CLI:
- "show ssl ech [<name>]" displays all ECH configurations or a specific
  one.
- "add ssl ech <name> <payload>" adds a new PEM-formatted ECH
  configuration.
- "set ssl ech <name> <payload>" replaces all existing ECH
  configurations.
- "del ssl ech <name> [<age-in-secs>]" removes ECH configurations,
  optionally filtered by age.