…FEC mode is supported before setting it

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**What I did**
Expose supported FEC modes to `STATE_DB.PORT_TABLE|<port>.supported_fecs`.
The orchagent calls `get_port_attribute` to get attribute `SAI_PORT_ATTR_SUPPORTED_FEC_MODE` during initialization and then records it into internal data.
   - By default, the supported FEC modes will be returned by SAI and exposed to `STATE_DB`. Eg. `rs,none` means only `rs` and `none` is supported on the port.
     - The orchagent will check whether the FEC mode is supported on the port before calling the SAI API to set it.
     - The CLI will check whether the FEC mode is in `supported_fecs` before setting FEC mode on a port to the `CONFIG_DB`
   - In case the SAI API does not support any FEC mode on the port, `N/A` will be exposed to `STATE_DB`
     - The orchagent will deny any FEC setting and prints log.
     - The CLI will deny FEC setting.
   - In case the SAI API `get_port_attribute` returns `Not implemented`
     - No check will be performed before setting a FEC mode to SAI on the port.
     - The CLI will check whether the FEC mode is defined before setting it to `CONFIG_DB`.

**Why I did it**
It is not supported to set FEC mode on some platforms. To avoid error, we need to expose the supported FEC list.

**How I verified it**
Manually test and mock test.

**Details if related**

Signed-off-by: Stephen Sun <[email protected]>

Signed-off-by: Stephen Sun <[email protected]>