Support OTN attenuator and amplifier

[jjin62]

1. Add a new precision attribute flag for optical analog values (e.g., gain, tilt, power), instead of using double/float point type which would introduce NaN issue.
2. Add support for optical attenuator in experimental extensions, derived from openconfig-optical-attenuator.yang.
3. Add support for optical amplifier in experimental extensions, derived from openconfig-optical-amplifier.yang.

[jjin62]

1. Introducing two new SAI objects OTN-OA and OTN-ATTENUATOR using SAI experimental extension mechanism. OA and ATTENUATOR are optical modules which can form a simplest optical amplifier.
2. Gauged/Analog values (optical power, gain, attenuation etc..) are usually represented in a float point format (double or float). However, double/float data type is prohibited in SAI to avoid NaN issue (see meta/Makefile compile flag "-Wfloat-equal"). Therefore, a @precision tag is introduced to represent the number of digits after the decimal point. Any integer type with a @precision tag can be used to represent a decimal type.
   Ref SAI Support optical transport device - proposed by sonic-wg-otn #2216

[azure-pipelines]

Commenter does not have sufficient privileges for PR 2217 in repo opencomputeproject/SAI

[kcudnik]

{ to new line, and add empty libe before return same in line 434

[jjin62]

done.

[kcudnik]

there should be test in saisanitycheck.c for that value if it's in reasonable range, like 0..18 etc since you can put number like 23423542542 and it will be cast as int, truncated, to some absurd value which will make no sense

[jjin62]

good point. check precision range [0..18], 18 is the max precision used in transponder.